1. Field
An embodiment of the present invention relates to a pulse measurement device, method and medium, and more particularly, to a device, method and medium measuring a pulse by controlling pressure applied to a photoplethysmography (PPG) sensor and an amount of light applied to a PPG sensor according to an exercise activity level of a user.
2. Description of the Related Art
Recently, due to a heightened awareness of fitness, a growing number of people are now concerned with living a healthy lifestyle. Adequate nutrition and frequent exercise are known to be a necessary component of the healthy lifestyle. However, when a person excessively exercises without consideration for his or her body condition, problems may occur.
Accordingly, an exercise management method that assists a user in maintaining the appropriate level of exercise for the user's current fitness level is needed. A method of recognizing and managing an exercise state of the user by measuring a bio-signal such as a pulse rate or a heart rate is a representative example.
In this regard, a photoplethysmograph (PPG) is a device that detects the perfusion of blood through tissue by shining light through it. PPG works by shining infrared light through a capillary bed. As arterial pulsations fill the capillary bed, the changes in volume of the vessels modify the absoption, reflection, and scattering of the light. Although PPG accurately indicates the timing of events such as heart rate, it is sensitive to motion artifacts.
In a conventional pulse measurement method using a PPG signal, a pulse rate before exercise and a pulse rate after exercise are measured and compared with each other. This makes continuous or real-time analysis impossible.
Another conventional pulse measurement method uses an electrocardiogram (EKG). Since several electrodes are attached to the chest of a user using a chest-band to measure the EKG, it is similarly inconvenient and impracticable to analyze data in real-time.
Generally, in the conventional PPG measurement method, as shown in FIG. 1A, a measurement device is attached to bare skin such as on an end of a finger or toe of a user. Infrared light is generated by an LED 110, which is a light source of a probe 100, as shown in FIG. 1B. There, a first light sensor 120 senses an amount of light reflected after penetrating the skin and being absorbed into erythrocytes flowing in a blood vessel. A second light sensor 130 senses the amount of infrared light penetrating the blood vessel. The detected light appears as waves as shown in FIG. 1C, and whether the blood circulates well is determined by analyzing the detected waves.
As shown in FIG. 2, a peak of a PPG signal in a rest state 210 is continually generated, while having a slight delay from an R-peak of an EKG signal, and has the same frequency component in a fast Fourier transform. As shown in FIG. 2, the generated EKG signal and the generated PPG signal are the same in the rest state 210, which includes a resting state and a walking state of up to 3 km per hour.
However, in a walking state 220 at a speed of 6 km per hour or a running state 230 at a speed of 9 km per hour, the frequency of the PPG signal is consistent with a frequency Z_ACC generated by an acceleration sensor rather than a frequency of the EKG signal, as shown in FIG. 2.
As described above, because the conventional PPG sensor is increasingly affected by the activity level of the user, as the user activity level increases, the frequency of the detected PPG signal becomes more consistent with the frequency of the acceleration signal.
In addition, referring to FIG. 3, as a measurement point is changed from a first point P1 to a second point P2, corresponding to the change in position of sensors S and D from 311 and 312 to 311′ and 312′ respectively, a PPG signal can no longer be detected reliably.
Generally, as shown in FIG. 4, when measuring a pulse, since a maximum peak value of a PPG signal is generated at a point (Pt=Pi−Po=0) in which inner pressure Pi of a blood vessel is identical with external pressure Po of the blood vessel, with the external pressure Po being an optimum pressure 510 for sensing the PPG signal.
Referring to FIG. 5, in the conventional PPG sensor, the optimum PPG measurement pressure point changes from points/areas 510 to 520 due to instability of a sensor position as movement increases with the user's increased activity. This makes detecting a clear PPG signal difficult.
Accordingly, since noise and interfering light, generated when a user exercises, interferes with the precise measuring of a pulse, a technology for measuring a pulse by stably detecting a PPG signal is desired.